Telomeres, the specialized protein-DNA structures present at the chromosomal terminus serve as 'caps' to prevent deleterious processes that would result in toss of linear chromosomes. The maintenance of an appropriate length of telomedc simple sequences to provide cap function is therefore essential for cell viability. Indeed, defects in telomere size control have been linked with both aging and oncogenesis. We have been studying the mechanism of size control in the yeast Saccharomyces cerevisiae as a model system. The basic outline of telomere size control is remarkably similar in yeast and vertebrates. Hence, the construction of a model system that can be easily manipulated gives us the opportunity to provide the framework for higher eukaryotic functional homologs. Previous studies have uncovered a process termed telomeric rapid deletion (TRD) that excises over-elongated telomere tracts to wild type sizes by a recombination mechanism. Two components regulate this process: the mechanism for the deletion process, and the mechanism of sizing among telomeres that governs the precision of deletion. The major goal in this proposal is to understand the mechanism of TRD. Three aims will be pursued. The first aim is the characterization of the cell cycle control of TRD and the characterization of meiotic TRD which displays dramatically increased rates of TRD. The only protein known to be essential for TRD is the recombination protein Mrel 1. The second aim is to study the essential role of Mrel 1 in TRD through the extensive characterization of a battery of missense alleles falling within unique domains of the protein The third aim examines an unusual allele of Mrel 1, A470T, that confers two major phenotypes, a loss of TRD precision and an ability to bypass senescence conferred by a loss of telomerase. Specific hypotheses f or the function of Mrel 1 in checkpoint control on telomere processing are tested using the tools made available from preliminary data. Proteins that interact with the motif surrounding A470T will be sought through the use of the Tap-tag method of isolating soluble native complexes.